I would like you to be aware of the fact that these references are not reliable. It's an idea to always check The Scots Peerage, particularly Volume 1 with regards to Lady Barbara Fitzroy. It is very premature to name the father- John Churchill 1st Duke of Marleborough. There is no reliable source to substantiate this.
The Scots Peerage is a very reliable tool, as such, to use.
Here is a link to a page, which mentions the mistresses of King Charles ll. Please see "item ll" for Lady Barbara Fitzroy. King Charles ll has in fact acknowledged her as his illegitimate daughter. https://archive.org/details/scotspeeragefoun01pauluoft/scotspeerage...
I hope this helps
